Electric automobiles, hybrid automobiles and fuel cell vehicles employing a motor to obtain a force driving them have a secondary battery mounted therein. An electric automobile employs electrical power stored in the secondary battery to drive the motor to drive the vehicle. A hybrid automobile employs electrical power stored in the secondary battery to drive the motor to drive the vehicle or allows the motor to assist the engine to drive the vehicle. A fuel cell vehicle employs electrical power provided by a fuel cell to drive the motor to drive the vehicle, and employs electrical power provided by the fuel cell and in addition thereto that stored in the secondary battery to drive the motor to drive the vehicle.
Such vehicles have a regenerative braking function. More specifically, when the vehicle is braked, the motor is functioned as a power generator to convert the vehicle's kinetic energy to electrical energy to brake the vehicle. The obtained electrical energy is stored to the secondary battery and reused for example in accelerating the vehicle.
Excessively discharging and charging the secondary battery impairs its performance as a battery. Accordingly, the secondary battery's state of charge (SOC), also referred to as “available capacity,” must be considered in controlling charging/discharging the secondary battery. In particular, a hybrid automobile of a type of a vehicle that has a heat engine mounted therein and uses the heat engine to drive a power generator to generate power which is in turn stored in a secondary battery, is often controlled to allow the secondary battery to have an SOC between a fully charged state (or 100%) and a completely uncharged state (or 0%), i.e., around 50 to 60%, to allow the secondary battery to accept regenerated power and also supply power to the motor upon receiving a request to do so. Accordingly, the secondary battery's available capacity (or SOC) must be detected more accurately.
One such method of detecting a secondary battery's available capacity is a method of doing so as based on the voltage appearing at a terminal of the secondary battery, i.e., open circuit voltage, electromotive force. Furthermore, as the terminal's voltage varies with a current's value, there is also a method employing a current sensor to measure a value of a current charging/discharging a secondary battery, and obtaining an integral of the value to estimate available capacity.
Japanese Patent Laying-Open No. 2003-149307 discloses a method of calculating an available capacity of a battery that can ensure high precision in estimating an SOC without depending on the battery's charging/discharging pattern. This method employs the battery's electromotive force and an integral of the value of a current to estimate the battery's available capacity (or SOC) and includes the steps of determining a correction parameter for the SOC by the battery's electromotive force and employing the correction parameter to correct an SOC obtained from the integral. The step of employing includes the steps of obtaining the electromotive force from the battery's voltage; obtaining a first estimated SOC from the electromotive force; obtaining a second estimated SOC from the integral; obtaining an amount for correction from a difference between the first and second estimated SOCs with the correction parameter; and employing the amount for correction to correct the second estimated SOC.
In accordance with this method the correction parameter is determined so that the battery's SOC obtained from the integral is corrected from the battery's electromotive force in an increased amount for an SOC range for which an SOC is estimated from the electromotive force with high precision and in a decreased amount for an SOC range for which an SOC is estimated from the electromotive force with low precision. Thus the SOC can be estimated with improved precision for all SOC ranges and there can be provided a method of calculating an available capacity of a battery with high precision without relying on its charging/discharging patterns.
As described in Japanese Patent Laying-Open No. 2003-149307, however, the correction parameter is determined by the electromotive force. More specifically, the correction parameter is determined to be large for a range for which an SOC would be estimated as based on the electromotive force with high precision, whereas it is determined to be small for a range for which an SOC would be estimated from the electromotive force with low precision. In such a method if the secondary battery is charged/discharged with a current having a small value and accordingly an SOC estimated from electromotive force is poor in precision, the correction parameter may nonetheless be determined to have a large value, which can result in an inaccurately calculated SOC.